1. Field of the Invention
The present invention relates to novel composite materials comprising a reinforced vitreous matrix, and to a process for the production thereof.
2. Description of the Prior Art
Composite materials comprising a vitreous matrix and, in particular, a reinforcing amount of fibers, are currently of great technical interest in light of their good thermomechanical properties. For this reason, they may advantageously be used in the aeronautical and aerospace fields for applications requiring good strength at intermediate temperatures, i.e., on the order of 600.degree. to 1,000.degree. C.
However, most of the materials of this type developed to date are not completely satisfactory, both relative to their final properties and to the processing thereof.
Among the most widely used and best performing composite materials comprising vitreous matrices, a distinction may be made between those having a matrix essentially of a borosilicate glass, with a B.sub.2 O.sub.3 content on the order of 10% to 30% by weight, and those having a matrix essentially based on silica, i.e., having a silica content greater than 95% weight.
The first type presents the major disadvantage of low corrosion resistance due to the presence of the B.sub.2 O.sub.3 oxide, which is particularly sensitive to hydrolysis, and the second type may have mechanical properties, particularly relative to impact strength, that are insufficient for certain applications.
The technique most typically employed for the production of such materials entails impregnating a fiber preform (reinforcing agent) with a slip containing, in various forms, all of the constituents required to provide the desired vitreous composition, then drying the thus impregnated preform.
The stages of impregnating and drying may be repeated until a prepreg is obtained having the desired amount by volume of fibrous reinforcement and/or a plurality of prepregs is stacked into an array and made integral by heating the dried prepregs at moderate temperatures in order to produce large size mono- or bidirectionally reinforced composites. Finally, the preform is densified in a compression stage at elevated temperatures. This latter stage, having in particular the purpose of making the glass flow through the strands of the preform, requires a relatively low viscosity of the glass (less than 10.sup.7 poises) and thus a high compression temperature.
Hence, to obtain suitable results relative to density and thermomechanical properties, it is known that borosilicate glasses (glass transition temperature Tg on the order of 530.degree. C.) must be compressed at temperatures of at least about 1,150.degree. to 1,200.degree. C., while for glasses based essentially on silica (Tg on the order of 850.degree. C.) this temperature is about 1,400.degree. to 1,600.degree. C. These high pressing temperatures entail the risk of damaging the fibers which constitute the reinforcing structure.
It is also known to this art that the limit on the temperatures of intended utilization of such composite materials having a glassy matrix is determined by the glass transition temperature of the glass and that, generally, this limit is approximately Tg+100.degree. C.; it is thus found that the application temperature of a composite having a borosilicate matrix is limited to 600.degree. C. and that of a composite having a silica matrix to 1000.degree. C.
It is thus seen that for the materials of the prior art there exists a very large differential between their application temperature, or temperature of intended utilization (T.sub.u) and the compression temperature (T.sub.p) that is required for the production thereof. This results in an appreciable reduction in their economic and/or industrial worth.